Hi there, sorry for the inconvenience! You can still configure a proxy but it'll have to be slightly more involved.
Open up your XRE/prefs.js file in your profile folder. You'll want to add entries for the preferences documented here:
So you'll want to add something that looks like:
You might need different options depending on what type of proxy you're using. Refer to the link above for more info on that.
We'll look into facilitating this better in a future update.